Continuous flow fuel control system

ABSTRACT

A fuel control system for providing steady state fuel flow to a gas-turbine engine comprises a pulse-width modulated, solenoid-operated fuel metering valve, an engine speed transducer, and a remote microprocessor communicating with the valve and the speed transducer through an optical fiber. The valve is operated by a direct current signal which is pulsed at a frequency substantially higher than the natural frequency of the valve spring/plunger system, whereby the plunger remains axially displaced from the valve seat to provide continuous flow through the valve. The amount of axial plunger displacement and, hence, the fuel flow rate are prescribed by the pulse width of the signal directed through the valve.

CROSS REFERENCE TO RELATED APPLICATIONS

The instant application is a continuation-in-part of U.S. patentapplication Ser. No. 07/225,193, filed July 28, 1988, entitled "FuelControl System", assigned to the assignee of the instant application,and now abandoned.

BACKGROUND OF THE INVENTION

The instant invention relates to a fuel control system providing steadystate fuel flow to a gas turbine engine of a guided missile in directproportion to engine fuel requirements. The system uses a pulse-widthmodulated, solenoid-operated fuel metering valve to provide asubstantially linear fuel flow rate in response to control signalsderived from engine parameters and communicated from a remote processorvia fiber optic link.

The prior art teaches the use of solenoid-operated valves wherein fluidflow through the valve is regulated by means of a magnetic plunger whichis normally mechanically biased to seat in a valve body orifice andwhich is cyclically unseated therefrom by energizing a solenoid coilwrapped around the valve body. The resulting pulsating flow createsdifficulties, in the form of stall or flameout, when used for supplyingfuel to a gas turbine engine requiring a continuous supply of fuel. InU.S. Pat. No. 4,015,426, Hobo attempts to mitigate the fuel flowdiscontinuities inherent to fuel systems employing such known pulsedsolenoid-operated valves by supplying thereto pulsed control signalshaving a constant frequency but with a variable pulse width. However,the modification of the pulses as taught in Hobo does not, in and ofitself, provide for continuous fuel flow, as the valve taught thereincontinues to operate in a pulsating manner; rather Hobo relies on theexternal mechanical damping provided by a elastic fuel line acting as ahydraulic accumulator in order to smooth the flow of fuel to the engine.Hobo further teaches the use of a second valve, 180 degrees out-of-phasewith the first valve, to double the effective frequency of the fuelpulse to further mitigate fuel flow discontinuity. The use of multiplevalves and external damping means nonetheless remain impractical giventhe space and cost constraints imposed by a guided missile systemapplication.

U.S. Pat. No. 3,523,676 to Barker teaches a fluid control valve whereina solenoid-operated plunger is offset from the central axis of the valveso as to induce radial vibration when the plunger is cycled into contactwith the valve seat. Such radial vibration is used to reduce theundesirable axial rebound inherent in valves which operate in theaforementioned cyclical fashion, whereby greater fuel flow accuracy isobtained. However, the reduction of axial rebound of the plungercorrespondingly further defines each fuel pulse generated by thecyclically-operating valve, thereby increasing the fuel flowdiscontinuities experienced by the engine.

Moreover, the Barker valve contemplates application in a stationaryenvironment, such as a chemical processing plant, where the correctivevibratory action induced by the eccentric plunger is not defeated byexternal vibratory sources. Thus, the Barker valve would not beeffective in an environment which itself is subject to severe vibratoryaction, such as within a launched missile.

Thus, in short, systems known to the prior art incorporating solenoidbased metering valves deliver fuel in a pulsating fashion and requireexternal damping and/or internal vibratory control to stabilize fuelflow to an engine. Systems of this sort are impractical for use insensitive fuel control applications such as that required for guidedmissiles in which space and cost considerations, as well as reliabilityof operation, are critical factors.

SUMMARY OF THE INVENTION

It is an object of the instant invention to provide a fuel controlsystem for a gas turbine-powered guided missile, the control of whichmay be accomplished remotely.

Another object of the instant invention is to provide a fuel controlsystem for a guided missile which employs a solenoid-operated valve tometer fuel in a proportional, steady state fashion to the enginethereof, as determined by operating parameters.

The fuel control system of the instant invention for metering fuel tothe engine of a guided missile comprises a fuel metering valve on themissile having a tubular valve body, the interior surface of whichdefines a passage extending therethrough, and a magnetic plunger locatedwithin the passage. The interior surface of the valve body is providedwith a radially-tapered portion defining the valve seat. The plunger isalso provided with a radially-tapered end portion to facilitatefluid-tight engagement between the plunger and the valve seat. A valvespring mechanically biases the plunger towards the valve seat while anO-ring seated into an annular groove in the valve seat ensures theformation of a seal between the plunger and the valve seat when theplunger is maximally biased thereagainst.

A solenoid coil is wrapped around the valve body which, when energizedby a constant frequency pulsed direct current signal, produces a biasingmagnetic field which axially displaces the plunger to allow fuel flow.The width of the signal pulses prescribes the axial displacement of theplunger relative to the valve seat and, hence, the instantaneous rate offuel flow through the valve. Significantly, the frequency at which thesignal pulses are directed through the solenoid coil is substantiallyhigher than the natural frequency of the valve spring/plunger assembly,whereby a dynamic equilibrium is achieved within the valve so as tomaintain the plunger in a substantially fixed position within thepassage away from the valve seat. Started another way, the mechanicalbiasing provided by the spring in combination with the inertia of theplunger act to mechanically rectify the effect of the pulsating magneticfield generated by the solenoid coil. As a result, the valve operates ina manner similar to a servomechanism, with the plunger being displacedto and maintained in a position corresponding to the average power inputrepresented by the signal pulses directed therethrough, wherebycontinuous fuel flow is achieved. The instant fuel control system thusobviates the need for the vibratory control and external dampingrequired in prior art systems.

Preferably, the optimal instantaneous pulse width is determined fromengine parameters in a closed loop control system. In the preferredembodiment, a microprocessor generates the fuel flow commands whichdefine the signal pulses to be directed through the solenoid coil whilereceiving critical operating data, such as engine speed, from one ormore sensors on the missile to provide immediate corrective action. Forexample, where a speed sensor is employed to monitor the rotationalspeed of the engine, the microprocessor compares the output from thespeed transducer with a speed set point and accordingly adjusts thewidth of the signal pulses directed through the solenoid coil.Instantaneous airframe speed and altitude, for example, may also besupplied to the microprocessor for use in adjusting the width of thesignal pulsed directed through the solenoid coil.

The preferred embodiment of the instant invention further provides forthe remote positioning of the microprocessor relative to the missile,thereby obviating the need for providing an expendable microprocessorfor each missile. Specifically, the ground-based microprocessorcommunicates with the missile-based speed transducer and solenoid coilvia a fiber optic cable. The fiber optic link enables the transmissionof the high frequency signal pulses required for controlling fueldelivery to the high-speed gas turbine engines utilized in guidedmissiles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic schematic of a proportional flow fuel controlsystem for a guided missile featuring remote processor control inaccordance with the instant invention; and

FIG. 2 is an enlarged cross-sectional view of the fuel metering valve ofthe fuel control system shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

Referring to FIG. 1, an exemplary fuel control system 10 constructed inaccordance with the instant invention regulates the flow of fuel 22 froma fuel tank 18 through fuel lines 12 and 14 to a gas turbine engine 16providing propulsion for a guided missile (not shown). The fuel tank 18is held at constant pressure by compressor bleed air 20 from the engine16 to assist fuel flow through the fuel lines 12 and 14 and a fuelmetering valve 24. A speed transducer 26 generates an output 78proportional to the rotational speed of the engine 16. The output 78 ofthe speed transducer 26 is communicated to a ground-based microprocessor28 via an optical fiber 30 and two signal converters 32 and 34. Themicroprocessor 28 compares the output of the speed transducer 26 to aset point 72 provided by a missile operator and generates a fuel flowcommand 74 which, subsequent to conversion by signal converters 32 and34, comprises a pulse-width modulated direct current signal 66 for usein controlling the valve 24, as described more fully below. In thismanner, the speed transducer 26 provides feedback information to themicroprocessor 28 regarding engine speed to facilitate precise controlthereof.

More specifically, FIG. 2 shows the fuel metering valve 24 of theinstant fuel control system 10 as having a tubular valve body 31 with aninput port 33 at one end thereof communicating with the fuel tank 18 viafuel line 12 and an output port 35 at the other end thereofcommunicating with the engine 16 via fuel line 14. The interior surface36 of the valve body 31 defines a passage 38 extending therethrough inthe direction of fuel flow.

A plunger 40 with a magnetic core 42 is located within the valve passage38. The magnetic core 42 acts as a solenoid core 42, with the plunger 40moving along the central axis 44 of the valve body 31. A solenoid coil46 is wrapped around the valve body 31 so as to be concentric with thepassage 38 therein.

The interior surface 36 of the valve body 31 has a radially-taperedportion 48 defining the valve seat. The plunger 40 has aradially-tapered end portion 50 to facilitate fluid-tight engagement ofthe plunger 40 with the valve seat 48. An O-ring 52 is seated into anannular groove 54 in the valve seat 48 to further ensure a seal betweenthe plunger 50 and valve seat 48 when the plunger 40 is mechanicallybiased thereagainst by a spring 56 engaging with a surface 58 of theplunger 40 and a surface 60 of a cap 62. The cap 62 has threads 64 whichallow for the spring 56 to be pre-loaded when the cap 62 is screwedtight.

The plunger 40 is electromagnetically biased away from the valve seat 48by the interaction of a magnetic field with the magnetic core 42 of theplunger 40. The biasing magnetic field is generated when the solenoidcoil 46 is energized by the pulse-width modulated signal 66 generated bythe microprocessor 28. Preferably, the rise time of the magnetic pulsegenerated by the solenoid coil 46 is long in comparison to the inertiaof the plunger 40. A pulse frequency is also selected which issubstantially higher than the natural frequency of the plunger/valvespring assembly so as to minimize plunger 40 travel when the solenoidcoil 46 is energized by pulses of a varying width, thus simulating aservomechanism response from the valve in which plunger 40 is opened toa position corresponding to the average power input represented by thepulses. A steady state condition is achieved wherein the plunger 40vibrates slightly about the point of average power as related to signalpulse width. The pulse width is selected in proportion to desired enginespeed and thus fuel requirements. Wider pulses cause the plunger 40 tobecome further axially displaced from the valve seat 48, therebyallowing more fuel 22 to flow through the gap 68 between the O-ring 52and plunger 40 and causing a greater engine speed. Similarly, narrowpulses generate reduced fuel flow and, hence, a lesser engine speed.

As noted hereinabove, the microprocessor 28 regulates fuel flow throughthe valve 24 by generating a fuel flow command 74 which, subsequent toconversion by signal converters 32 and 34, provides a pulse-widthmodulated direct current signal 66 for energizing the solenoid coil 46of the valve 24. Under the instant invention, the microprocessor 28 maybe located on the missile or remotely. In the preferred embodimentillustrated in FIG. 1, a ground-based microprocessor 28 communicateswith the missile-based speed transducer 26 and solenoid coil 46 via theoptical fiber 30 extending between the microprocessor 28 and themissile. Specifically, the digital fuel flow commands 74 generated bythe microprocessor 28 in response to engine speed feedback 78 from thespeed transducer 26, as well as inputs 72 from the ground-based missileoperator, are encoded to a digital word and transmitted through thefiber optic cable 30 as an optical signal 76 by signal converter 32, andthen detected and decoded in the missile to provide the pulse-widthmodulated signal 66 for operating the valve 24. Engine speed feedback 78from the speed transducer 26 on the missile's engine 16 is likewisedigitally encoded and transmitted in the opposite direction through thefiber optic cable 30 as an optical signal 80 by signal converter 34, andthen detected and decoded by ground-based signal converter 32 for use bythe microprocessor 28.

The simultaneous transmission of data in both directions through thefiber optic cable 30 is achieved by using encoders and decoders withineach signal converter 32 and 34 capable of operating on two distinctwavelengths. Thus, for example, the output 78 from the speed transducer26 is converted to a green light by signal converter 34 for transmissionthrough the fiber 30 for ultimate use by the microprocessor 28, and thefuel flow commands 76 generated by the microprocessor 28 are transmittedthrough the fiber 30 to the missile as yellow light for ultimate use bythe valve 24. The specific wavelengths employed for optical signals 76and 80 are selected so as to minimize signal attenuation in the fiberoptic cable 30, in the manner known to one skilled in the art.

The fiber optic link allows for the transmission of the fuel flowcommands from the ground-based processor to the missile at the highfrequency and with the accuracy required for optimum performance. In thepreferred embodiment, the digital words generated by the microprocessor28 are transmitted through the fiber optic cable 30 at a frequency ofbetween 60 to 100 Hz to control engine speed within a ±3 percent speedtolerance bandwidth. It will be readily appreciated, however, thathigher transmission rates are possible to more tightly regulate enginespeed, due to the broad bandwidth of the fiber optic cable 30.

The instant invention permits the repeated use of microprocessor 28which is part of computer facilities (not shown) at a remote location,thereby providing a cost savings over missiles having an on-board fuelcontrol microprocessor which is lost on any missile flight. It isfurther noted that the output port 35 of the valve 24 of the instantfuel control system 10 may be directly connected to the engine 16, i.e.,without the aid of fuel-pulse dampening hoses, due to the continuousfuel flow provided thereby.

While the preferred embodiment of the invention has been disclosed, itshould be appreciated that the invention is susceptible of modificationwithout departing from the spirit of the invention or the scope of thesubjoined claims. For example, the pulse-width modulated valve 24on-board the missile may be replaced by a rotary valve operated by aproportional rotary solenoid or a variable-speed pump. Thesolenoid-operated rotary valve or variable speed pump on-board themissile is controlled by the remote processor 28 by using a pulse-widthmodulated signal as described hereinabove or a variable voltage DCsource. The instant invention also contemplates the use of avariable-opening rotary valve operated by a stepper motor under remoteprocessor control to precisely and continuously meter fuel to themissile's engine.

I claim:
 1. A fuel control system for regulating fuel flow to an enginecomprisinga fuel tank; a fuel metering valve comprisinga tubular valvebody with an input port at one end and a tapered output port at anopposite end; a magnetic plunger axially aligned with the central axisof said valve body, said plunger having a tapered end portioncomplimentary to the tapered end of said valve body; resilient means forbiasing said plunger towards the tapered end of said valve body; anelectromagnet for biasing said plunger away from the tapered end of saidvalve body so as to control fuel flow through said valve body from saidinput port to said output port; and control means for supplying directcurrent pulses to said electromagnetic biasing means, said pulses beingof variable duration but of a constant frequency substantially higherthan the natural frequency of said plunger under the bias of saidbiasing means.
 2. The fuel control system according to claim 1 includingsealing means disposed between the tapered interior surface of saidvalve body and the radially tapered end portion of said plunger, saidsealing means comprising an O-ring seated in an annular groove in thetapered portion of said valve body.
 3. The fuel control system accordingto claim 1 wherein said control means comprisesa speed transducer onsaid engine generating an output proportional to the rotational speedthereof; a microprocessor, in communication with said speed transducer,for generating fuel flow commands responsive to said output of saidspeed transducer; and means for generating said signal pulses from saidfuel flow commands.
 4. The fuel control system according to claim 3wherein said control means further comprisesa fiber optic cable; firstmeans for encoding and transmitting said fuel flow commands along saidfiber optic cable; first means for detecting and decoding said encodedfuel flow commands for use in generating said signal pulses; secondmeans for encoding and transmitting said speed transducer output alongsaid fiber optic cable; and second means for detecting and decoding saidencoded output of said speed transducer transmitted along for use bysaid microprocessor.